The present invention relates to airfoil assemblies such as wings or stabilizers having movable control surfaces such as ailerons, flaps, elevators, and the like. The invention relates more particularly to such a control surface that is designed to create an air flow slot between the control surface and the fixed airfoil to which it is attached when the control surface is deflected away from a nominal position, so that air can flow through the air flow slot and thereby reduce the tendency for air to separate when flowing over the deflected control surface.
An aircraft wing requires two primary aerodynamic characteristics to perform well on takeoff. First, the wing must have sufficient lift at a given angle of attack, and second, the wing must have a high lift-to-drag ratio during the climb segment of the takeoff maneuver. These two characteristics can often be in opposition.
To obtain sufficient lift at a given angle of attack, it is often necessary to employ a trailing-edge flap on a wing. A flap is an airfoil-shaped member pivotally attached to the trailing edge of the wing and extending along at least part of the wing""s span. Deflecting the flap downward effectively increases the camber or curvature of the wing, which increases the lift of the wing at a given angle of attack. Flaps typically are limited in span because of the requirement for other devices such as ailerons on the outboard portions of the wings. Conventional ailerons are similar in structure to flaps, but their primary purpose is to provide directional and attitude control to the aircraft rather than to augment the lift of the wings.
While lowering the flaps substantially increases the wing lift, it also results in increased drag. The aerodynamic drag of the wing comes from several sources, but one important source called xe2x80x9cinducedxe2x80x9d drag has its genesis in non-uniform distribution of the aerodynamic lift along the span of the wing. It is not possible to have uniform lift along the span of the wing, because the lift must fall to zero at the tip of the wing. Accordingly, induced drag is an inevitable component of the overall wing drag. It is known, however, that induced drag can be minimized or reduced by designing the wing to have a spanwise lift distribution that is as close as possible to a theoretical optimum lift distribution having a generally elliptic shape. When flaps are deflected, the lift is increased over the part of the span having the flaps, while the outboard portions of the wings not having the flaps do not have augmented lift. The lift distribution therefore is driven away from the optimum elliptic shape, resulting in increased induced drag.
To counteract this effect, deflected ailerons have been used to add lift to the outboard portions of the wings during takeoff in an attempt to drive the lift distribution closer to the theoretical optimum. Traditional ailerons of the sealed, simple-hinged type can undergo only moderate deflections before they begin to separate, and therefore can add only a small amount of outboard lift before flow separation begins. Further deflection of the ailerons beyond this point may actually add more drag as a result of flow separation than is saved by reducing the induced drag, such that the overall drag may rise.
It is known that the angle to which an aileron can be deflected without separating can be substantially increased by providing an air flow slot between the fixed wing and the aileron such that air can flow through the slot. This type of aileron is sometimes referred to as a xe2x80x9cgappedxe2x80x9d aileron. When a gapped aileron is deflected trailing-edge down, air flows from the lower surface of the wing through the air flow slot to the upper surface of the wing and over the upper surface of the aileron, which tends to prevent flow separation on the aileron.
Flow through the air flow slot is not desirable in cruise flight conditions because it can impair the aerodynamic efficiency of the wing. Accordingly, movable trailing-edge control surfaces have been developed that have an air flow slot when the control surface is deflected but wherein the air flow slot is closed when the control surface is in an undeflected or nominal position such as during cruise flight. To this end, a door along one surface of the wing is pivotable from a nominal position closing the air flow slot when the surface is undeflected, to a position opening the air flow slot when the control surface is deflected toward the surface of the wing having the door.
Prior gapped trailing-edge control surface mechanisms of this type have tended to be quite complicated, employing a large number of moving parts, which leads to increased cost and complexity of assembly and maintenance. Some of these devices have employed cam tracks and cam followers such as rollers for mechanically linking the door to the control surface so that deflection of the control surface causes the door to open. Such cam track and roller arrangements are subject to wear and can bind as a result of slight misalignments that can occur in service. Some prior gapped trailing-edge control surfaces have had the pivot for the control surface located outside the aerodynamic contour of the wing, which causes increased aerodynamic drag during all phases of flight. Additionally, in some prior devices, the opening of the door in response to deflection of the control surface has not been sufficiently rapid, such that less than an optimal amount of air flow through the air flow slot has been achieved.
The present invention addresses the above needs and enables other advantages. In accordance with one aspect of the invention, an airfoil assembly, such as a wing or horizontal stabilizer, comprises a fixed airfoil and a movable control surface connected to the trailing edge of the fixed airfoil, the control surface being rotatable about a pivot relative to the fixed airfoil. The control surface is spaced rearwardly from the trailing edge of the fixed airfoil such that an air flow slot is defined between the fixed airfoil and the control surface. The airfoil assembly also includes a movable door connected to the fixed airfoil proximate a first aerodynamic surface thereof, the door having a forward edge and an aft edge that respectively form substantial seals with the airfoil and the control surface when the control surface is in a neutral position so as to close off the air flow slot. The assembly further includes a linkage connecting the door to the control surface. The linkage is structured and arranged such that when the control surface is pivoted in a first direction to move the trailing edge of the control surface generally toward the first aerodynamic surface, the door is caused to rotate about a pivot spaced from the aft and forward edges of the door in such a manner that openings for air flow through the air flow slot are created between the aft edge of the door and the control surface and between the forward edge of the door and the airfoil. The openings thus enable a desirable amount of air flow to suppress flow separation on the control surface.
Preferably, the pivot about which the control surface rotates is located inside the external aerodynamic contour of the fixed airfoil. Thus, there are no external pivots and associated fairings giving rise to added drag of the airfoil.
The airfoil in one embodiment includes a panel connected to the fixed airfoil on an opposite side thereof from the door. The panel closes the air flow slot when the control surface is in its neutral position and when the control surface is pivoted away from the neutral position in a second direction opposite from the first direction that causes the door to open, but allows the air flow slot to open when the control surface is deflected in the first direction. The panel allows the control surface to be deflected to larger angles in the second direction.
In accordance with another aspect of the invention, the linkage connecting the control surface to the door is free of rolling or sliding elements, consisting exclusively of members pivotally connected to one another. This arrangement has advantages in terms of ruggedness and reduced wear compared to prior gapped control surface mechanisms employing cam tracks and rollers.
More particularly, the linkage includes a door control arm arranged to rotate along with the control surface about the pivot thereof. The door control arm is connected to a crank to which the door is attached, and the crank is pivotal about a fixed pivot. The linkage further comprises a drive link connected between the door control arm and the crank, a bracket rigidly affixed to the door, and a stabilizing link. One end of the stabilizing link is connected to the bracket and the stabilizing link is rotatable about a fixed pivot longitudinally offset from the fixed pivot for the crank. The crank is rotatably connected to the bracket at a location longitudinally spaced from the one end of the stabilizing link. The fixed pivot for the stabilizing link is forward of the fixed pivot for the crank, and the one end of the stabilizing link is forward of the location at which the crank is connected to the bracket.
The control surface in one embodiment comprises an aileron for a wing, and has an advanced airfoil shape providing improved aerodynamic efficiency at low air speeds. The door controls the opening of the air flow slot at the lower surface of the wing, and the panel closes the air flow slot at the upper surface of the wing when the aileron is not deflected trailing-edge down. When the aileron is deflected trailing-edge down and air is flowing through the air flow slot onto the upper surface of the aileron, the air is accelerated to its maximum velocity where the maximum curvature exists on the aileron upper surface, which is located in the region of the air flow slot at the upper surface of the wing. It has been found that this allows the leading-edge pressure peak to be driven to a lower coefficient of pressure without the aileron separating, such that the aileron can be deflected to greater trailing-edge down angles before separation begins. The aileron therefore can be used to provide significant lift enhancement for the outboard portion of the wing.
The airfoil in accordance with the invention can be a main wing for an aircraft, or an auxiliary airfoil such as a horizontal stabilizer or the like. The control surface can be an aileron, a flap, an elevator, or the like.